1. Field of the Invention
The present invention relates to a pressure compensated electromagnetic proportional directional flow control valve used in a liquid-pressure system.
2. Description of the Related Art
Japanese Laid-Open Patent Application Publication No. 2000-120913 discloses an electromagnetic proportional directional flow control valve configured to drive a directional flow control spool by a solenoid to control a flow direction and flow rate of pressure oil. A casing of the electromagnetic proportional directional flow control valve includes: a liquid-pressure supply port to which the pressure oil is introduced from an oil-pressure pump; a first external connection port communicated with one of oil chambers of an oil-pressure cylinder; a second external connection port communicated with the other oil chamber of the oil-pressure cylinder; and a return port communicated with a tank. Then, the flow direction of the pressure oil between the first external connection port and the second external connection port can be switched by causing the directional flow control spool to move in the casing, and the flow rate of the pressure oil can be controlled by changing the opening degree of an aperture formed by a gap between the casing and the directional flow control spool. Further, the electromagnetic proportional directional flow control valve includes a pressure compensated valve provided between the directional flow control spool and the return port. The pressure compensated valve acts to maintain constant a pressure difference between before and after the aperture in the electromagnetic proportional directional flow control valve even if load pressure of the oil-pressure cylinder fluctuates. Thus, the pressure compensated valve compensates so as to maintain a constant flow rate of the pressure oil discharged from the oil-pressure cylinder.
Regarding the pressure oil discharged from the oil-pressure pump in the above electromagnetic proportional directional flow control valve, some pressure oil necessary for driving the oil-pressure cylinder is introduced to the first external connection port or the second external connection port, and the surplus pressure oil needs to be released to the other line. Typically, a relief valve configured to determine maximum pressure of the entire circuit is provided on a passage extending between the oil-pressure pump and the liquid-pressure supply port. In a case where the oil pressure in the passage reaches the maximum pressure, the relief valve opens to release the pressure oil in the passage to the tank. Thus, the circuit is protected.
In a case where only the relief valve is provided on the passage extending between the oil-pressure pump and the liquid-pressure supply port, the surplus pressure oil is always released to outside through the relief valve. Therefore, the discharge pressure of the oil-pressure pump always increases up to a set pressure of the relief valve. On this account, an external oil-pressure source uneconomically increases in pressure, and this deteriorates the circuit efficiency. A typical solution to this problem is that a bleed valve is provided on the passage extending between the relief valve and the liquid-pressure supply port, and the discharge pressure necessary for driving the oil-pressure cylinder is maintained without uneconomically increasing the discharge pressure of the oil-pressure pump. However, in the case of additionally providing the bleed valve in the oil-pressure system, the number of components, the plumbing, and the size of the device increases.